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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>

<span class="keyword">class</span> <span class="identifier">none_t</span> <span class="special">{/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/};</span>

<span class="keyword">inline</span> <span class="keyword">constexpr</span> <span class="identifier">none_t</span> <span class="identifier">none</span> <span class="special">(/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/);</span>

<span class="special">}</span> <span class="comment">// namespace boost</span>
</pre>
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<p>
          Class <code class="computeroutput"><span class="identifier">none_t</span></code> is meant to
          serve as a tag for selecting appropriate overloads of from <code class="computeroutput"><span class="identifier">optional</span></code>'s interface. It is an empty,
          trivially copyable class with disabled default constructor.
        </p>
<p>
          Constant <code class="computeroutput"><span class="identifier">none</span></code> is used to
          indicate an optional object that does not contain a value in initialization,
          assignment and relational operations of <code class="computeroutput"><span class="identifier">optional</span></code>.
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